Laser and light beam intrusion detectors are now employed extensively to protect against unauthorized intrusion into secured areas. Such detectors usually take the form of interferometers employing interference effects to detect disturbances. For example, it has been proposed to utilize a fiber loop with counter-propagating beams, and detect phase changes between the two beams and between one of the beams and a reference beam in order to detect a disturbance on the fiber loop. It was also proposed that a frequency or phase modulator could be inserted in the apparatus to improve sensitivity. (See U.S. Pat. No. 4,885,462 issued to Dakin, and U.S. Pat. No. 5,012,088 issued to Cole, et al.) It has also been suggested to propagate two light beams in the same direction in two fibers, and utilize one beam to measure a physical variable and the other beam as a reference. (See U.S. Pat. No. 5,004,913 issued to Kleinerman.) Use of two overlapping fiber loops has also been proposed to determine the position of any disturbance. (See U.S. Pat. No. 5,355,208 issued to Crawford et al.) Backscattered light in combination with a Mach-Zehnder interferometer may also be used for intrusion detection. (See U.S. Pat. No. 5,194,847 issued to Taylor et al.) Fiber optic vibration detectors have been suggested employing Michelson type interferometers with one fiber used to detect the vibration and another fiber used as a reference. (See U.S. Pat. No. 5,381,492 issued to Dooley et al.)
Secure information transmission systems have also been proposed employing a Mach-Zehnder or Sagnac interferometer where at least one of the light paths has a phase or path length modulator. (See U.S. Pat. No. 5,140,636 issued to Albares.) A random path length modulator may be inserted in the loop. (See U.S. Pat. No. 5,694,114 issued to Udd.)
For systems detecting physical intrusion in a secured place, it may be possible for an intruder to defeat the system by diverting the optical beam and injecting a substitute interference pattern. For high security applications, systems are often supplemented by using multiple beams and other detection schemes, which can be fairly costly.
It is desirable, therefore, to provide a low cost intrusion detector which is not vulnerable to beam diversion.